Diaphrgam construction for differential pressure transducer

ABSTRACT

A diaphragm construction for a differential pressure transducer wherein the diaphragm is deflected under pressure, and which has means around the periphery thereof to isolate the diaphragm stresses from the mounting portion of the diaphragm.

Unlted States Patent 1191 1111 3,793,885

Frick Feb. 26, 1974 DIAPHRGAM CONSTRUCTION FOR 3,618,390 11/1971 Frick73/398 c DIFFERENTIAL PRESSURE TRANSDUCER 313601664 12/1967 Straube I3,505,875 4/1970 Benner, Jr 73/407 R [75] Inventor: Roger L. Frick,Minneapolis, Minn.

[7 3] Assignee: Rosemount Inc., Eden Prairie, Mich. primwy 1 1d 0 w i 22Filed: Sept. 5, 1972 HEX:.,,. 1, PK 7 .sfifff'f i fiiE,999

Westman [21] Appl. No.: 286,287

52 us. c1. 73/398 c, 73/407 R [57] ABSTRACT Int. CL A diaphragmconstruction for a differential pressure Field of Searchm 398 407 92/98,transducer wherein the diaphragm is deflected under 100 pressure, andwhich has means around the periphery thereof to isolate the diaphragmstresses from the [56] Referelwes Clted mounting portion of thediaphragm.

UNITED STATES PATENTS 3,195,022 7/1965 Werner et a] 73 393 c 12 3 DrawmgF'gures DIAPHRGAM CONSTRUCTION FOR DIFFERENTIAL PRESSURE TRANSDUCERBACKGROUND OF THE INVENTION 1. Field of the Invention.

The present invention relates to the diaphragm construction for pressuretransducers.

2. Prior Art.

The present application illustrates the improvements in the diaphragmconstruction over my U.S. Pat. No. 3,618,390, issued Nov. 9, 1971. Thedifferential pressure transducer functions are clearly explained in thispatent, and the diaphragm shown in the patent is a membrane typediaphragm that is supported against overpressure by internal surfaces ofthe pressure cell. This same overpressure support is used with thepresent invention, but the improvements of the diaphragm constructionpermit the use of a thicker diaphragm so that higher pressure ranges canbe sensed, while the adverse effects of very high bending moments in thediaphragm adjacent the mounting edges are reduced.

Insofar as prior art diaphragm constructions are concerned, U.S. Pat.No. 3,505,875, issued to Benner, Jr. on Apr. 14, 1970 shows a diaphragmwhich is intended to isolate the main portion of the diaphragm from themounting edge. However, the configuration of the diaphragm in thispatent is rather complex, and difficult to manufacture. Also, U.S. Pat.No. 3,360,664, issued to Straube on Dec. 26, 1967 shows a diaphragm usedin an accoustic radiating transducer and which is designed with ahinging section to attempt to isolate the main diaphragm'from themounting edges. Again, the particular shape used with this diaphragmdoes not lend itself well to an application as a differential pressuretransducer.

As a matter of general interest, U.S. Pat. No. 2,161,980 issued to Rungeet al. on June 13, 1939 shows an elastically oscilating oscillator whichis a plate-like device having reduced cross sectional areas between thecenter portions and the outer portions thereof.

SUMMARY OF THE INVENTION The present invention relates to a differentialpressure transducer diaphragm, wherein the diaphragm has a reduced crosssectional area web portion adjacent its mounting edges to isolate thebending stresses in the diaphragm from the peripheral mounting ring ofthe diaphragm. The present diaphragm as shown is used in a capacitancepressure sensor wherein pressure will deflect the diaphragm toward oraway from at least one stationary capacitor plate formed in a surface ofthe pressure transducer. In the form shown, the stationary capacitorplate has a surface shape conforming to the deflected surface shape ofthe diaphragm under pressure, and the stationary capacitor plate surfaceis positioned to substantially fully support the diaphragm when thediaphragm is subjected to its maximum rated pressure so thatoverpressures will not destroy or damage the diaphragm. The diaphragm isthus supported fully on the stationary capacitor plate under anyoverpressure without overstressing the diaphragm.

With the present diaphragm construction, the diaphragm can be made tooperate satisfactorily over a high pressure range without becomingexcessively stressed. The web portion that connects the outer mountingring of the diaphragm to the central portion is more highly stressedthan a plate diaphragm of uniform thickness would be, but the overallperformance of the diaphragm construction is considerably improved overa uniform thickness diaphragm. This is due to the isolation of stressesfrom the mounting ring, and the resultant minimization of the hysteresisand instability of the diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a vertical sectional viewthrough a sensing portion of a typical differential pressure transducerutilizing a diaphragm made according to the present invention;

FIG. 2 is a vertical sectional view of a diaphragm made according to thepresent invention with parts broken away from the center portion; and

FIG. 3 is a vertical sectional view of a modified form of the diaphragmof FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT My U.S. Pat. No. 3,618,390 forDifferential Pressure Transducer, issued Nov. 9, 1971 shows a typicaldifferential pressure transducer in which the diaphragm of the presentinvention may be used. Therefore, the construction of the outer housingof the pressure transducer, and the mounting of the sensing cell and theconnection for sensing circuitry is omitted, and that portion of thespecification of U.S. Pat. No. 3,618,390 is incorporated herein byreference.

FIG. 1 of the present application corresponds generally to FIG. 4 ofU.S. Pat. No. 3,618,390 with the new sensing diaphragm construction inplace, and generally includes the sensing cell 10 which comprises alarge metal housing 11, which is made into two sections, 11A and 118that are placed together along a parting line and which are separated bya measuring or sensing diaphragm 12 made according to the presentinvention. The metal sections 1 1A and 118 have cup like, circular crosssection cavities 13 which are filled with a rigid insulation material,such as a glass or ceramic material. The insulation filling thesecavities is placed into the cavities 13 and fused to the metal walls ofthe sections 11A and 118. Because the pressure transducer cellillustrated is used with isolating diaphragms that are in contact withthe sensed fluid, and the sensing diaphragm 12 is isolated, the cavitysections 11A and 118 each have passageways leading therethrough from theinterior to shallow cavities 14 on the opposite sides of each of thehousing sections from the glass filled cavities 13. A separate ceramictube 15 having a plurality of axially extending passageways 16 ispositioned in each of the passageways in the housing sections 11A and118 before the glass filling the cavities 13 is fused into place, andthese passageways 16, in connection with an opening 17, providepassageways for fluid or liquid to move between cavities l4 and thepressure sensing chambers formed by diaphragm 12.

Small metal tubes 20 are placed through provided openings in each of thehousing sections 11A and 11B, and are fused into place with the ceramicfilling material in the cavities 13. The tubes 20 and the ceramic tubes15 extend out beyond the glass filling when the cavities 13 are filled(the housing is still in two sections at this stage), and the metaltubes 20 also extend out beyond the peripheral wall of the housingsections.

Each of the individual housing sections 11A and 11B is ground off and aconcave surface which is, in part, substantially spherical is groundinto the surface of each of the housing sections. These concave surfacesare shown in FIG. 1, and after the shape has been made to correspond tothe deflection curvature of the diaphragm 12, the concave surfaces arecovered with suitable thin metal coatings forming capacitor platesindicated at 21 and 22, respectively. The metal tubes 20 areelectrically connected to their respective capacitor plates and formelectrical leads for the capacitor plates 21 and 22, which form thestationary capacitor plates for the capacitance type differentialpressure sensing cell shown. The passageways 16 in tubes open throughthe capacitor plates, and the openings or passageways of the tubes alsoopen through the capacitor plates.

After the two housing sections 11A and 11B have been individuallyprepared, the metal diaphragm 12 is placed between the sections, and iswelded into place with a continuous peripheral bead weld 23 which joinesthe two housings sections 11A and 11B together and holds the diaphragm.The weld 23 also hermetically seals the chambers on the interior of thehousing defined between the stationary capacitor plates 21 or 22, andthe diaphragm 12.

First and second isolation diaphragms 24 and 25, respectively, are thenwelded into place around the peripheries of the depressions or cavities14 formed in each of the housing sections. The isolation diaphragms canbe made of thin stainless steel and are corrugated so they will flexeasily. The outer surfaces of these isolation diaphragms are open to thepressure to be sensed, through a housing which is shown in U.S. Pat. No.3,6l8,390.

When assembled, the sensing diaphragm 12 and the isolation diaphragms 24and 25 form two chambers which are sealed sensing chambers comprising afirst chamber 26, and a second chamber 27. These sensing chambers 26 and27 are filled with a liquid, which may be a silicone base oil, throughthe respective tubes 20,

and then the tubes 20 are sealed at their outer ends.

The tubes 20 are used as leads in the capacitor plates after thechambers have been filled. The electrical lead to the diaphragm 12,which is the movable capacitor plate, can be connected to the housing11. Because the chambers 26 and 27 are completely filled and also thepassageways 16 in the tubes 15, and the chambers 14, any differentialpressure on the isolation diaphragms 24 and 25 will cause a deflectionof the main sensing diaphragm 12 with respect to one of the other of thestationary capacitor plates 21 or 22. When initially filled, and withthe isolation diaphragms at equal pressure, the diaphragm 12 will extendsubstantially straight and midway between the stationary capacitorplates 21 and 22.

The unit is constructed so that the stationary capacitor plates 21 and22 are conformed generally to the deflection curvature of the mainsensing diaphragm l2, and are positioned so that they will substantiallyfully support the diaphragm 12 across its entire working surface whenthe diaphragm 12 is subjected to its maximum rated pressure.

It can be seen that the main sensing diaphragm 12 is supported at itsouter edge and it is supported all around the periphery. Referring nowto FIG. 2, a diaphragm made according to the present invention is shownand includes a center sensing portion 30.

The center portion 30 of the sensing diaphragm 12 is circular in planview. Also, the outer periphery of the diaphragm 12 is circular. Asshown, the diaphragm 12 has an outer mounting ring 31 that is connectedto the sections 11A and 118 at its outer edges with the weld 23, andthis ring 31 (which is an annular ring) is connected to the centersensing section 30 with a thin web 32. When the diaphragm center portion30 deflects under pressure, the radial bending stresses at the outercorner where edge surface 30A joins the sensing surface of the centerportion 30 will be substantially zero and therefore the main centerportion 30 performs nearly equal to a true free edge diaphragm in itspressure deflection characteristics. The free edge diaphragm ischaracterized by having a zero radial bending moment at its edge. Thecondition permitting the center portion 30 to act as a free edgediaphragm is accomplished when the slope of the deflection curve at thecorner where edge surface 30A joins the sensing surface of centerportion 30 (or the angle of edge surface 30A with respect to a referenceplane parallel to the undeflected diaphragm) is substantially the sameas the slope of the deflection curve of the web 32 where it joins edgesurface 30A, under the pressure loading to the entire diaphragmassembly.

Thus, by having an outer mounting ring 31 that is attached to thehousing, and having a thin web 32 of some radial length joining theouter ring 31 and the center diaphragm portion 30, the center diaphragmportion 30 is substantially isolated from its mounting ring so thathysteresis and instability effects are minimized. The bending moments atthe outer mounting edges are substantially reduced, and the centerportion 30 acts like a free edge" diaphragm.

In FIG. 3, a modified sensing diaphragm I2 is shown. In this particularform, the sensing diaphragm 12 has a center sensing portion 35, amounting ring 36, and a joining web portion 37. The web portion 37 isnonuniform in thickness, varying from its greatest thickness T at itsouter edges where it joins ring 36, to a minimum thickness 1 whereitjoins the center portion 35. The corners where the edge surface 35A ofthe center diaphragm portion 35 meets the sensing surfaces of the centerportion 35A also move like a free edge diaphragm, and the tapered orvariable thickness web 37 provides a section where the bending stressesare more evenly distributed over the length (radial length) of the webso that maximum stress is lower than would be found in the uniformthickness web 32 of FIG. 2. The design shown in FIG. 3 is preferred forhighest pressure ranges, but is more complex to manufacture. It doeshave the advantage of reducing the level of maximum stress and thereforeincreasing the pressure range can be employed with the sensing diaphragmassembly 12 while retaining the benefits of the center portion 35behaving substantially like a free edge diaphragm, and also isolatingthe clamped ring 36 from the main diaphragm portion 35.

With a radial length L" for the webs, as shown in FIGS. 2 and 3, and atypical diaphragm diameter (D) of one and one-half inches, the D/L mayvary between 6 and 60. The ratio of L/T is typically greater than 0.5and less than 5.0. For the design of FIG. 3, the ratio of T/t is equalto or greater than 1, and preferrably no greater than 4.

It should be noted that the diaphragm constructions of FIGS. 2 and 3permit use of the diaphragm in differential configuration, or in otherwords where the deflection characteristics are substantially the same inboth directions of deflection of the diaphragm in use.

It should be noted that the edge surfaces A and A are cylindrical andform right angles with the sensing surfaces of the center portions 30and 35. The web joins the edge surfaces with only a very small fillet orradius.

What is claimed is:

l. A pressure transducer of the capacitance type having a housingincluding a surface defining a first capacitor plate, and a diaphragmmounted to be deflected under pressure with respect to said firstcapacitor plate; said diaphragm comprising a central sensing portioncomprising a disc member having a sensing surface and a bisectingreference plane generally parallel to the sensing surface, a peripheralmounting ring symmetrically located with respect to said bisecting planeand fixedly, rigidly mounted on said housing, and an annular web portionextending radially to join said ring and said central portion; said webbeing substantially thinner than the central portion at the junction ofthe web and central portion, said web portion extending symmetrically indirection of said bisecting plane and being defined by generally flatsurfaces on opposite sides of said bisecting plane, said flat surfacesbeing of length in direction between the ring and the central portion inrelation to the size of the central portion so that the central portiondeflection under pressure substantially corresponds to the deflection ofa free edge diaphragm.

2. The pressure transducer of claim 1 wherein said housing comprises ametal structure, and wherein the ring is welded to the housing toprevent movement of the ring relative to the housing during deflectionof said central portion.

3. The pressure transducer of claim 1 wherein the central portion has aperipheral edge surface, and the major portion of the edge surface ofthe central portion is substantially perpendicular to the sensingsurface of the diaphragm and said edge surface is joined to the flatsurfaces of said web with small fillets.

4. The pressure transducer of claim 1 wherein said surface defining saidfirst capacitor plate is configured to substantially conform to theshape of said diaphragm as said diaphragm is deflected toward said firstcapacitor plate, said first capacitor plate being spaced from thediaphragm a distance whereby said central portion is supportedsubstantially across its entire sensing surface by said first capacitorplate when said diaphragm is subjected to a preselected maximumpressure.

5. The pressure transducer of claim 1 wherein said housing isconstructed in two sections, said first capacitor plate being positionedon a first housing section, and a second surface of a second housingsection defining a second capacitor plate, said diaphragm being mountedto said housing between said first and second capacitor plates, andmeans to permit subjecting said diaphragm to differentials in pressureon its opposite side surfaces.

6. The pressure transducer of claim 1 wherein said central portion iscircular, and has a diameter D, and said web extends in radial directionbetween the central portion and said ring a distance L, and wherein theratio D/L ranges between 6 and 60.

7. The pressure transducer of claim 6 wherein said web extends in radialdirection a distance L, and said web is of a substantially uniformthickness t, and wherein the web is constructed to a proportion wherebythe ratio L/T is greater than 0.5 and is less than 5.0.

8. The pressure transducer of claim 6 wherein the web extends annularlyaround the central portion and increases in thickness in radiallyoutward direction from a thickness t adjacent the edge of said centralportion to a thickness T adjacent said ring and the ratio of T/t isequal to or greater than 1 and is no greater than 4, with T being nogreater than the thickness of the central portion.

9. A pressure transducer having a housing, a diaphragm mounted to bedeflected under pressure, means to sense deflection of said diaphragmunder pressure; said diaphragm comprising a disc shaped central sensingportion, a peripheral mounting ring fixedly, rigidly mounted on saidhousing and surrounding said disc shaped central portion, and a webportion extending radially from the central portion and joining saidring and said central portion; said web portion being substantiallythinner than the central portion where the web and central portion joinand being of length in radial direction between the ring and the centralportion in relation to the size of the central portion so that thecentral portion deflection under pressure substantially corresponds tothe deflection of a free edge diaphragm, said web portion being notsubstantially thinner throughout its radial outward length than it iswhere it joins said central portion.

10. The pressure transducer of claim 9 wherein said web portionincreases in thickness in direction radially outwardly from the centralportion to the ring.

11. The combination as specified in claim 9 wherein said disc shapedcentral sensing portion has a sensing surface, and an annular edgesurface substantially perpendicular to said sensing surface, surroundingsaid disc shaped central portion, and said web portion being defined bysubstantially planar surfaces, at least one of which joins said edgesurface of said central portion with a small fillet.

12. The combination as specified in claim 9 wherein said disc shapedcentral sensing portion has at least one sensing surface, and areference plane parallel to said sensing surface and passing through atleast portions of said central portion, said web being defined by a pairof generally radially extending fiat surfaces positioned symmetricallywith respect to said plane, and extending between said peripheralmounting ring and said central sensing portion.

1. A pressure transducer of the capacitance type having a housingincluding a surface defining a first capacitor plate, and a diaphragmmounted to be deflected under pressure with respect to said firstcapacitor plate; said diaphragm comprising a central sensing portioncomprising a disc member having a sensing surface and a bisectingreference plane generally parallel to the sensing surface, a peripheralmounting ring symmetrically located with respect to said bisecting planeand fixedly, rigidly mounted on said housing, and an annular web portionextending radially to join said ring and said central portion; said webbeing substantially thinner than the central portion at the junction ofthe web and central portion, said web portion extending symmetrically indirection of said bisecting plane and being defined by generally flatsurfaces on opposite sides of said bisecting plane, said flat surfacesbeing of length in direction between the ring and the central portion inrelation to the size of the central portion so that the central portiondeflection under pressure substantially corresponds to the deflection ofa free edge diaphragm.
 2. THe pressure transducer of claim 1 whereinsaid housing comprises a metal structure, and wherein the ring is weldedto the housing to prevent movement of the ring relative to the housingduring deflection of said central portion.
 3. The pressure transducer ofclaim 1 wherein the central portion has a peripheral edge surface, andthe major portion of the edge surface of the central portion issubstantially perpendicular to the sensing surface of the diaphragm andsaid edge surface is joined to the flat surfaces of said web with smallfillets.
 4. The pressure transducer of claim 1 wherein said surfacedefining said first capacitor plate is configured to substantiallyconform to the shape of said diaphragm as said diaphragm is deflectedtoward said first capacitor plate, said first capacitor plate beingspaced from the diaphragm a distance whereby said central portion issupported substantially across its entire sensing surface by said firstcapacitor plate when said diaphragm is subjected to a preselectedmaximum pressure.
 5. The pressure transducer of claim 1 wherein saidhousing is constructed in two sections, said first capacitor plate beingpositioned on a first housing section, and a second surface of a secondhousing section defining a second capacitor plate, said diaphragm beingmounted to said housing between said first and second capacitor plates,and means to permit subjecting said diaphragm to differentials inpressure on its opposite side surfaces.
 6. The pressure transducer ofclaim 1 wherein said central portion is circular, and has a diameter D,and said web extends in radial direction between the central portion andsaid ring a distance L, and wherein the ratio D/L ranges between 6 and60.
 7. The pressure transducer of claim 6 wherein said web extends inradial direction a distance L, and said web is of a substantiallyuniform thickness t, and wherein the web is constructed to a proportionwhereby the ratio L/T is greater than 0.5 and is less than 5.0.
 8. Thepressure transducer of claim 6 wherein the web extends annularly aroundthe central portion and increases in thickness in radially outwarddirection from a thickness t adjacent the edge of said central portionto a thickness T adjacent said ring and the ratio of T/t is equal to orgreater than 1 and is no greater than 4, with T being no greater thanthe thickness of the central portion.
 9. A pressure transducer having ahousing, a diaphragm mounted to be deflected under pressure, means tosense deflection of said diaphragm under pressure; said diaphragmcomprising a disc shaped central sensing portion, a peripheral mountingring fixedly, rigidly mounted on said housing and surrounding said discshaped central portion, and a web portion extending radially from thecentral portion and joining said ring and said central portion; said webportion being substantially thinner than the central portion where theweb and central portion join and being of length in radial directionbetween the ring and the central portion in relation to the size of thecentral portion so that the central portion deflection under pressuresubstantially corresponds to the deflection of a free edge diaphragm,said web portion being not substantially thinner throughout its radialoutward length than it is where it joins said central portion.
 10. Thepressure transducer of claim 9 wherein said web portion increases inthickness in direction radially outwardly from the central portion tothe ring.
 11. The combination as specified in claim 9 wherein said discshaped central sensing portion has a sensing surface, and an annularedge surface substantially perpendicular to said sensing surface,surrounding said disc shaped central portion, and said web portion beingdefined by substantially planar surfaces, at least one of which joinssaid edge surface of said central portion with a small fillet.
 12. Thecombination as specified in claim 9 wherein said disc shaped centralsensing portion has at least one sensing surface, and a reference planeparallel to said sensing surface and passing through at least portionsof said central portion, said web being defined by a pair of generallyradially extending flat surfaces positioned symmetrically with respectto said plane, and extending between said peripheral mounting ring andsaid central sensing portion.